Su Lin's Birthday Cake: Raw Video

Researchers said one cub is a boy who tipped the scales at 6 ounces. They don’t know the other cub’s gender or weight because mama bear hasn’t put it down since birth, the China News Service reports.

Su Lin (which translates to “a little bit of something very cute”) was born at the San Diego Zoo in August 2005. Her mother, 22-year-old Bai Yun, still lives at the zoo and is the oldest actively breeding female in captivity. Bai Yun has given birth to six giant panda cubs: Hua Mei, Mei Sheng, Su Lin, Zhen Zhen, Yun Zi and Xiao Liwu.

San Diego's Cutest Critters

The San Diego Zoo’s giant pandas are on a research loan from the Chinese government, which can recall the pandas to their native land after they turn 3 years old. Su Lin returned to China in 2010.

So who is the panda twins’ father? That’s not so black and white. Su Lin mated naturally with Yuan Yuan, but was then artificially inseminated by Lu Lu, according to the China News Service. The cubs are expected to take a paternity test in a year or two.